When an audio signal including speech is sent from a transmitting device to a receiving device, the speech intelligibility may be affected by background noise from both the transmitting and receiving sides. For example, when listening to a telephone phone call via a hearing aid, the audio signal transmitted by the telephone is mixed with an environmental sound signal sensed by the microphone of the hearing aid to maintain the hearing aid wearer's awareness of surroundings. Because the level of the received speech and the level of noise at the receiving side may vary dynamically, there is a need to maintain speech intelligibility as well as listening comfort for the listener continuously and automatically.